1. Field of the Invention
The present invention relates to oil field tools. More specifically, the invention relates to a system for and a method of using cutting tools disposed in wellbores.
2. Background of the Related Art
Historically, oil field wells are drilled as a vertical shaft to a subterranean producing zone forming a wellbore, the wellbore is lined with a steel tubular casing, and the casing is perforated to allow production fluid to flow into the casing and up to the surface of the well. In recent years, oil field technology has increasingly used sidetracking or directional drilling to further exploit the resources of productive regions. In sidetracking, a slot or xe2x80x9cwindowxe2x80x9d is cut in a steel cased wellbore typically using a mill and drilling is continued at angles to the vertical wellbore. In directional drilling, a wellbore is cut in strata at an angle to the vertical shaft typically using a drill bit. The mill and the drill bit are rotary cutting tools having cutting blades or surfaces typically disposed about the tool periphery and in some models on the tool end.
FIG. 1 is a schematic cross sectional view of a typical vertical wellbore 10. A casing 12 is disposed in the wellbore with a cutting tool 14, such as a mill, having cut a portion of a window 16 in a sidewall of the casing. The cutting tool 14 can be coupled to tubing 32, such as coiled tubing or a drill string, by a motor 19 having a shaft 18 that rotates the cutting tool. In such instance, the shaft 18 is known as an output shaft. Alternatively, the cutting tool can be coupled to a shaft 18 that forms a portion of a drill string that is attached to a surface rig. A motor disposed on the surface rig rotates the drill string which rotates the cutting tool and cuts the casing or other downhole components.
To direct the cutting tool 14 toward the side of the casing 12, a whipstock 22 is inserted into the wellbore. The whipstock 22 is used to direct the cutting tool or other tool in a direction that is angularly offset to the original wellbore by using a whipstock face 24, that is, a sloped surface which progressively narrows the open cross sectional area in the casing 12. The whipstock 22 is set in position in the casing at a given depth and the cutting tool 14 engages the whipstock face 24 as the cutting tool traverses downward. The cutting tool 14 is progressively deflected laterally toward the casing 12 as the cutting tool cuts the window 16. After the window 16 is cut and the cutting tool is removed, the whipstock 22 can remain in position to guide subsequent operations, such as directional drilling with drill bits.
FIG. 2 is a schematic cross sectional view of a cutting tool 14 coupled to the motor 19 at joint 26. The motor 19 includes an output end 46 and a shaft 18, where the motor transmits torque to the cutting tool 14 through the shaft 18. The cutting tool 14 is coupled to an end 34 of the shaft 18 at an engagement section 36 internally disposed in a bore 39 of the cutting tool 14. The shaft 18 has threads 35 which engage corresponding threads 37 on the cutting tool 14. A portion of the end 34 of the shaft 18 is surrounded by a peripheral wall 40 of the cutting tool disposed upstream from the engagement section 36 and defines an annular space 42, known as a box relief. The shaft 18 has a hexagonal shaft portion 52, which provides engagement surfaces for a wrench (not shown). By convention, an end 44 of the peripheral wall 40 is typically aligned with the downstream end 53 of the shaft portion 52, leaving exposed a portion of the shaft 18. A passageway 48 is formed in the shaft 18 and the cutting tool 14, where the passageway allows fluid to flow through the shaft and the cutting tool and then to exit through nozzles 50 in the cutting tool for washing away the debris as the cutting tool is rotated.
FIG. 3 is a schematic cross sectional view through the shaft, showing the peripheral wall 40 of the cutting tool surrounding the shaft 18 and the shaft portion 52. The peripheral wall 40 disposed about the perimeter of the shaft 18 defines the annular space 42.
One challenge with cutting a window with a mill or drilling an angled wellbore with a drill bit is the stress imparted to the cutting tool 14 and the shaft 18. The stress imparted from cutting the side of the casing 12 for a mill or the strata for a drill bit is not evenly displaced about a circumference of the rotating components. For instance, as best seen in FIG. 1, at joint 26 defining the connection between the shaft and the cutting tool, a first portion 28 of the joint 26 on the side of the cutting tool that cuts the window is placed under a longitudinal compressive load, but the portion 30 of the joint 26 that is opposite the window 16 is placed under a longitudinal tensile load. As the joint rotates, each portion 28, 30 is subjected to alternating compressive and tensile stresses. Additionally, the stresses on the joint 26 are proportional to the distance between the cutting surfaces of the cutting tool and the joint. A longer distance proportionally increases the stresses. The alternating stresses, especially using long cutting tools, create cyclical bending of the members, such as the cutting tool 14 and the shaft 18, and can produce stress fatigue and failure of one or more of the members. It is believed that at least a portion of the failures are due to stress concentrations in a stress failure region 41 near an upstream end of the threads 35 on the shaft 18. The downtime can be costly for retrieving a broken shaft 18 that involves fishing the parted assembly from the wellbore, replacing the shaft and reinserting the assembly down the wellbore.
There remains a need for an improved system and method for using a cutting tool at an angle in a wellbore, particularly for stiffening a cutting tool system to avoid the cyclical bending.
The present invention generally provides a system and method for stiffening a cutting tool assembly used in cutting laterally relative to a wellbore axis to reduce stresses and cyclical bending of the cutting tool assembly during cutting. The system includes a cutting tool attached to a shaft such as an output shaft of a motor or a drill string. A sleeve is disposed in an annular space, known as a box relief, defined between the shaft and a peripheral wall of the cutting tool. The sleeve is preferably fixed in the annular space by a sleeve ring surrounding a recess in the shaft, but can be coupled to the peripheral wall and/or shaft by, for example, a threaded engagement. As the cutting tool attempts to bend at a connection with the shaft during cutting, creating stresses at the connection, the stresses are distributed throughout the increased contact area of the sleeve with the cutting tool, causing less stress per unit area and distributing at least a portion of the stress away from the threaded engagement between the shaft and the cutting tool. The reduced stresses cause less fatigue and thus lower failure rate of the members. Also, the walls of the cutting tool surrounding the shaft are lengthened to engage even more surface area of the sleeve and further reduce the bending stresses. Also, the distance between the cutting portion of the cutting tool and an engagement portion between the cutting tool and the shaft can be shortened to reduce stresses on the engagement portion by forming a shorter cutting tool.
In one aspect, the invention provides a window milling system, comprising a shaft, a mill coupled to the shaft having walls at least partially surrounding a portion of the shaft. defining an annular space between the walls and the shaft, and a sleeve disposed in the annular space. The sleeve is preferably a split sleeve that fits snugly in the annular space. In another aspect, the invention provides a method of cutting a casing with a window mill system, comprising engaging a shaft with a mill, coupling the shaft to a rotatable member, placing the mill downhole in a wellbore, cutting a portion of a casing disposed in the wellbore with the mill, causing bending stresses on the mill, and at least partially distributing the bending stresses onto a sleeve disposed in an annular space between the mill and the shaft. A whipstock can be used to direct the mill laterally into the casing. In another aspect, the invention provides a cutting tool system, comprising a shaft, a cutting tool coupled to the shaft having at least one peripheral wall at least partially surrounding a portion of the shaft and defining an annular space between the wall and the shaft, and a sleeve disposed in the annular space. In another aspect, the invention provides a method of using a cutting tool, comprising engaging a shaft with a cutting tool, coupling the shaft to a rotatable member, placing the cutting tool downhole in a wellbore, causing bending stresses on the cutting tool, and at least partially distributing the bending stresses onto a sleeve disposed in an annular space between the cutting tool and the shaft.